With improved technology and usability of thermal cameras, they are being increasingly used in various industrial condition and quality control applications. Conventionally, thermal imaging has been used for monitoring the condition of various apparatus components. Typical uses have included, for example, the monitoring of the condition of various engines and bearings at intervals. By means of thermal imaging, defects in components can be detected in a contactless manner, for example as an increase in the surface temperatures of the components.
In paper industry, thermal imaging has also been applied to measure properties of the fibre web to be manufactured and/or processed, various rolls, as well as reels formed of the web, by methods which will be discussed in more detail below.
Article “Infrared Thermography—An Aid to Solving Paper Machine Moisture Profile Problems” (D. E. Vickery, J. E. Luce and W. Atkins, Tappi Journal, December 1978, Vol. 61, No. 12, pp. 17-20) discloses the use of a portable thermal camera for determining the moisture profile of the web in the cross-machine direction. According to the article, the temperature profile of the web in the cross-machine direction correlates clearly with the moisture profile of the web in the cross-machine direction, wherein for example wet streaks in the web in the machine direction are detected in a thermogram as streaks cooler than their environment. Said article also presents the use of a portable thermal camera for monitoring the condition of rotating machine components, such as rolls. In the determination of the temperature profile of said components in the cross-machine direction of the web, the temperature differences are detected as band-like temperature areas surrounding said rotating means.
The applicant's previous public patent application WO 00/45156 discloses a more sophisticated monitoring method, in which the monitoring of the moving web is implemented not only with a portable thermal camera but with one or more stationary thermal cameras for imaging the web in a continuous manner. Instead of control measurements to be taken at random or at regular intervals, the method described in WO 00/45156 is intended to be used for continuous monitoring of the web, particularly in connection with the coating of the web. A thermogram is displayed on one or more monitors to the user who can observe the process to be controlled substantially in real time, wherein, on the basis of phenomena detected in thermograms, the process to be controlled can be adjusted, if necessary, to optimize the process. The thermogram can also be recorded in a memory for later viewing.
However, the above-presented prior art methods utilizing a thermal camera involve the problem that the information contained in the thermal images is primarily analyzed by the user viewing the images. Thus, the user must have expertise and experience to find out the real cause of the phenomenon detected in the thermal images each time. For this reason, some of the information contained in the thermal images may also easily remain unutilized.
Furthermore, the solutions of prior art involve the problem that transient phenomena which may also occur only recurrently are easily disregarded by the user. In particular, phenomena which are very low in intensity and occur only recurrently in thermograms are very difficult to be detected by the human eye. In practice, the user will typically only see the average temperature profile of the object in the cross-machine direction in thermograms recorded according to prior art. For example, in the case of rolls, the problems are detected as band-like temperature areas surrounding the roll, and their cause cannot be found out on the basis of the measurement results in a fast and reliable way. In the machine direction, only such slow and trend-like temperature changes are detected, in which, for example, the temperature of the web at the measuring point is changed, for some reason, as a function of time in a more permanent manner.
To locate a malfunctioning component in the apparatus for manufacturing or processing the web, measurements according to prior art must be typically taken at several points in the process, which requires either the use of several thermal cameras placed in a stationary manner at different locations in the process, or taking the corresponding measurements by using a portable thermal camera. The first-mentioned method has the disadvantage of high equipment costs caused by the cameras, whereas the latter method has the disadvantage of slowness and poor repeatability of the manual measurements.